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GLYCERINE                                     4f»5

the free caustic alkali (see 2), the remainder being the percentage of Na.,0
as carbonate.

4.  Alkali combined with Organic Acids.—The percentage of Na.O
existing as organic salts is calculated by subtracting from the total Na,0
(i) the sum of the free Na20 (2) and the carbonated Na20 (3).

5.  Acidity.—Ten grams of the sample, dissolved in 50 c.c. of recently
boiled water, are titrated with normal caustic soda in presence of phenolph-
thalein.    The result is expressed as Na20 necessary to neutralise the acidity
of 100 grams of the glycerine.

6.  Residue at 160°.—In a 100 c.c. measuring flask, 10 grams of the

glycerine are weighed, diluted with a little water and treated with normal

acid or alkali (according as the sample is alkaline or acid) in such amount

that the glycerine assumes an alkalinity corresponding with 0-2% of Na20.

The liquid is then made up to volume and shaken, 10 c.c. (or, if the sample

is very impure, a lesser quantity sufficient to give a residue not exceeding

30-40 milligrams) being transferred to a tared porcelain dish 12 mm. deep

and with a flat base 6 cm. in diameter.   The bulk of the water is evaporated

off on the water-bath and the dish then placed in an air-oven (30 X 30 X 30

cm.), which is furnished with a thermometer, rests on an iron plate 20 mm.

thick, and has half-way up a shelf covered with asbestos board on which

the capsule containing the glycerine rests.   The latter is heated at 160°

until only traces of thin vapour are emitted, then removed from the oven,

allowed to cool, 0-5-1 c.c. of water added and the contents gently mixed.

The liquid is again evaporated on the water-bath and subsequently on the

oven at 160° until the residue, placed within the oven, no longer froths.

This operation usually requires 2-3 hours.

At this point the dish is kept in the oven at 160° for exactly one hour,
and is then removed, allowed to cool in a desiccator over sulphuric acid
and weighed. The residue is next treated with water, re-evaporated, dried,
kept at 160° for an hour as before, this procedure being repeated until the
loss occasioned does not exceed 1-1-5 mgrms. per hour.

The weight of the residue at 160° is corrected for the acid or alkali added
to bring the alkalinity to the desired point. With acid glycerine, 0-022
gram is subtracted for each c.c. of normal alkali added. With alkaline
glycerine the correction applied is that resulting from the transformation
of NaOH and Na,C03 into NaCl. The corrected weight gives the residue
at 160° and is calculated for 100 grams of the glycerine.

The residue is kept for the determination of any impurities capable of

acetylation.                                                                            ,.„

7. Organic Residue.-The organic residue represents the difference

between the residue at 160° and the ash.

It should, however, be noted that the C02 formed for the transformation
of organic acids during the incineration is not contained in the organic

aer.--On a clock-glass of about 15 c.c. capacity are placed 2-3
very voluminous asbestos, previously well washed with acid and
i wter and dried at 100°.   The whole is then le ft * ; a_
desiccator over sulphuric acid at a pressure of i-* .mm. of mercury until